Root Hydraulic Conductance in Epiphytic Orchids

Abstract

In tropical rainforests, rainfall quickly falls through the canopy where epiphytic orchids grow, exposing these plants to diurnal and/or seasonal episodes of ?drought.? Epiphytic orchids in particular have a specialized root structure derived from the epidermis called the velamen, which aids in rapid water uptake. The velamen structure is found in both aerial and embedded orchid roots. The purpose of this study is to compare root water uptake through the xylem by measuring root hydraulic conductance in three epiphytic orchids species with varying velamen layers: Dendrobium (6-8 velamen layers), Phalaenopsis (1-2 velamen layers), Oncidium (6-8 velamen layers). Root hydraulic conductance was compared in aerial and embedded roots of each species in order to determine if aerial roots are as efficient as embedded roots in taking up water. In addition, comparative anatomical data was used to explain observed differences in root hydraulic conductance. Physiological data collected shows a significantly higher root hydraulic conductance for species with more velamen layers (Dendrobium and Oncidium). Across all species, the embedded roots had a significantly higher root hydraulic conductance than aerial roots. Root anatomy of embedded roots showed the formation of root hairs as a result of substrate contact, allowing for more efficient water uptake. Further study should extend upon obtaining root hydraulic conductance in additional CAM vs. C3 orchid species with comparable velamen layers to discern efficiencies in water uptake for orchids using different photosynthetic pathways.